Codebook subset selection

ABSTRACT

The present invention provides for an improved application of signal strength weightings in a SDMA sectorized cellular network. The improved signal strength weightings application is conducted through the improved selection of weightings from a new codebook subset or by the selection of weightings from a larger codebook subset. In a further embodiment, an antenna beam index or bit map can be used to select the best beam(s) in a SDMA sectorized cellular network. In another embodiment, a field or factor in an uplink or downlink transmission packet can designate which directional transmission beam is best suited for the transmission or when the directional transmission beam should be activated.

CROSS-REFERENCE TO RELATE APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/146,764, filed Jan. 3, 2014, which is a continuation of U.S. patentapplication Ser. No. 13/758,446, filed Feb. 4, 2013 (now U.S. Pat. No.8,644,764), which is a continuation of U.S. patent application Ser. No.12/989,749, filed Oct. 26, 2010 (now U.S. Pat. No. 8,391,797), which isthe U.S. National Stage of International Application No. PCT/US09/02585,filed on Apr. 28, 2009, which claims the benefit of priority to U.S.Provisional Application No. 61/048,716, filed on Apr. 29, 2008. All ofthe above-identified applications are hereby incorporated by referencein their entireties as though fully and completely set forth herein.

FIELD OF THE INVENTION

A system and method for selection of codebook subset in a mobilecommunication system having multiple transmit antennas.

BACKGROUND OF THE INVENTION

There is an increasing demand on mobile wireless operators to providevoice and high-speed data services, and at the same time, theseoperators want to support more users per basestation to reduce overallnetwork costs and make the services affordable to subscribers. As aresult, wireless systems that enable higher data rates and highercapacities are needed. The available spectrum for wireless services islimited, and the prior attempts to increase traffic within a fixedbandwidth have increased interference in the system and degraded signalquality.

One problem exists when prior art omni-directional antennas are used atthe basestation because the transmission/reception of each user's signalbecomes a source of interference to other users located in the same celllocation on the network, making the overall system interference limited.Such an omni-directional antenna is shown in FIG. 1( a). In thesetraditional mobile cellular network systems, the base station has noinformation on the position of the mobile units within the cell andradiates the signal in all directions within the cell in order toprovide radio coverage. This results in wasting power on transmissionswhen there are no mobile units to reach, in addition to causinginterference for adjacent cells using the same frequency, so calledco-channel cells. Likewise, in reception, the antenna receives signalscoming from all directions including noise and interference.

An effective way to reduce this type of interference is to use multipleinput-multiple output (MIMO) technology that supports multiple antennasat the transmitter and receiver. For a multiple antenna broadcastchannel, such as the downlink on a cellular network, transmit/receivestrategies have been developed to maximize the downlink throughput bysplitting up the cell into multiple sectors and using sectorizedantennas to simultaneously communicate with multiple users. Suchsectorized antenna technology offers a significantly improved solutionto reduce interference levels and improve the system capacity.

The sectorized antenna system is characterized by a centralizedtransmitter (cell site/tower) that simultaneously communicates withmultiple receivers (user equipment, cell phone, etc.) that are involvedin the communication session. With this technology, each user's signalis transmitted and received by the basestation only in the direction ofthat particular user. This allows the system to significantly reduce theoverall interference in the system. A sectorized antenna system, asshown in FIG. 1( b), consists of an array of antennas that directdifferent transmission/reception beams toward each user in the system ordifferent directions in the cellular network based on the user'slocation.

The radiation pattern of the base station, both in transmission andreception, is adapted to each user to obtain highest gain in thedirection of that user. By using sectorized antenna technology and byleveraging the spatial location of mobile units within the cell,communication techniques called space-division multiple access (SDMA)have been developed for enhancing performance. Space-Division MultipleAccess (SDMA) techniques essentially creates multiple, uncorrelatedspatial pipes transmitting simultaneously through beamforming and/orprecoding, by which it is able to offer superior performance in multipleaccess radio communication systems.

This method of orthogonally directing transmissions and reception ofsignals is called beamforming, and it is made possible through advancedsignal processing at the base station. In beamforming, each user'ssignal is multiplied with complex weights that adjust the magnitude andphase of the signal to and from each antenna. This causes the outputfrom the array of sectorized antennas to form a transmit/receive beam inthe desired direction and minimizes the output in other directions,which can be seen graphically in FIG. 2.

While known methods exist in the conventional multi-user multipleantenna systems that employ an orthogonal precoder to place weightingson the spatially orthogonal beamforming transmissions, the known methodsand systems are not optimized in the precoding operations, and therebyfail to optimize the performance on the network. The present inventionresolves these problems. Further, the installation of many antennas atsingle base stations can have many challenges which are resolved by thepresent invention. Since the available spectrum band will probably belimited while the requirement of data rate will continuously increase,the present invention also supports an expansion of the availablespectrum over known methods for precoding in the cellular network.

The various components on the system may be called different namesdepending on the nomenclature used on any particular networkconfiguration or communication system. For instance, “user equipment”encompasses PC's on a cabled network, as well as other types ofequipment coupled by wireless connectivity directly to the cellularnetwork as can be experienced by various makes and models of mobileterminals (“cell phones”) having various features and functionality,such as Internet access, e-mail, messaging services, and the like.

Further, the words “receiver” and “transmitter” may be referred to as“access point” (AP), “basestation,” and “user” depending on whichdirection the communication is being transmitted and received. Forexample, an access point AP or a basestation (eNodeB or eNB) is thetransmitter and a user is the receiver for downlink environments,whereas an access point AP or a basestation (eNodeB or eNB) is thereceiver and a user is the transmitter for uplink environments. Theseterms (such as transmitter or receiver) are not meant to berestrictively defined, but could include various mobile communicationunits or transmission devices located on the network.

SUMMARY OF THE INVENTION

The present invention provides for an improved application of signalstrength weightings in a SDMA sectorized cellular network. The improvedsignal strength weightings application is conducted through the improvedselection of weightings from a new codebook subset or by the selectionof weightings from a larger codebook subset. In a further embodiment, anantenna beam index or bit map can be used to select the best beam(s) ina SDMA sectorized cellular network. In another embodiment, a field orfactor in an uplink or downlink transmission packet can designate whichdirectional transmission beam is best suited for the transmission orwhen the directional transmission beam should be activated.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention will become more readilyunderstood from the following detailed description and appended claimswhen read in conjunction with the accompanying drawings in which likenumerals represent like elements and in which:

FIG. 1 is a graphical illustration of an omni-directional antenna (a)and a sectorized antenna (b);

FIG. 2 is a graphical illustration of a weighted sectorized transmissionbeam directed to the desired user;

FIG. 3 is a graphical illustration of a multiple antenna transmissionsystem using precoding;

FIG. 4 is a codebook subset table for constant modulus;

FIG. 5 is a codebook subset table for antenna selection;

FIG. 6 is a precoding codebook subset table;

FIG. 7 is a precoding codebook subset table;

FIG. 8 is a precoding codebook subset table proposed in the presentinvention;

FIG. 9 is a precoding codebook subset table proposed in the presentinvention;

FIG. 10 is a larger precoding codebook subset table proposed in thepresent invention; and,

FIG. 11 is a precoding codebook subset table proposed in the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1( a), the overall transmission architecture 100 of anomni-directional antenna 105 that transmits radially outward equally invarious directions shown by arrows 125, 115, 135 and 140. The perimeterof the coverage area is shown by the area 120 for the transmissionarchitecture 100. Improved efficiencies have been achieved by using thesectorized antenna architecture 140 shown in FIG. 1( b).

Multiple antennas 145, 147 and 148 are shown in the architecture 140,wherein each antenna is directed toward a different region of thecellular network shown by the directional transmission 175 for coveragearea 150, transmission 190 for coverage area 157, and directionaltransmission 180 for coverage area 155. In this context, it is possiblefor system capacity to be improved by the sectorized architecture.

By weighting the various transmission signals, additional efficienciesand reduced interferences can be achieved as shown in FIG. 2 for thesectorized architecture 200. Multiple antenna 215, 220, 227 and 230direct transmissions (or receive transmissions) in the sectorizedantenna architecture 200. A directional antenna beam 235 is formed byscaling the signal with a set of weighting factors applied to an arrayof antenna elements, such as antenna element 230. The desired user 205is shown receiving a desired transmission 245 in coverage area 235,which is a heavily weighted transmission meant to be directed to thatuser 205. An interfering user 210 is shown with less weightedtransmission signals 240 to reduce the interference encountered by thatuser 210.

In FIG. 3, a precoding architecture 300 is shown where a data input 301is fed into the user selection component 310. The user selectioncomponent 310 sends the appropriate data through the appropriate datasignal line 315 to the precoding component 321. The appropriate data foreach user 350, 351, 352 may consist of channel encoded, interleaved,rate-matched, scrambled and/or modulated symbols. The precodingcomponent 321 provides an appropriate weighting for the signal strengthto be transmitted on the multiple antennas 320, 322 or 325. Based on thetargeted user 350, 351 and 352, the signal strength weighting of themultiple antennas to each of these targeted user will be adjusted toincrease the efficiency of the data transfer to the desired user andreduce interference with other users on the system.

The selection of specific codes to be used in the precoding component321 to provide appropriate weightings for the signal strength are shownin several tables documented in FIGS. 4-11. In FIG. 4, a constantmodulus 2-Tx codebook is shown, and in FIG. 5, an antenna selection 2-Txcodebook is shown. A codebook accepted under the TS 36.211 v8.2.0standard is shown in FIG. 6.

There are two possible configurations for the codebook selection usingthe codebooks at FIGS. 4, 5 and 6. In one configuration, the attachmentpoint (base station/antenna) may select one of the two subsets shown inFIG. 4 or 5 for use in a sector where the user is located. Theattachment point selects a subset codebook for all user equipment in thesame sector, such as using only the codebook shown in FIG. 4 or 5. Theattachment point selects the codebook subset for the user equipmentbased on some knowledge of the user equipment's channel condition. Thechannel condition information includes information regarding the userequipment's location information, the error rate for transmissions tothe user equipment, the number of re-transmissions to the userequipment, and the uplink sounding or other uplink transmissions, withthe uplink received beam-forming using a similar beam pattern as thatfor the downlink transmission.

In a second configuration, the user equipment can select the appropriatecodebook subset to be used in FIGS. 6, and the user equipment can selectbetween a total of 9 different distinct codewords for a 2-Tx twotransmission antenna system. The user equipment transmits an indicatorthat implicitly or explicitly indicates which codebook subset is chosen.The subset selection will be dictated in the second configurationthrough a higher layer activation depending on the codeword selectedfrom the codewords shown in FIG. 6, and the index of the selectedcodeword in the subset is signaled using 2 bits through the normal PMIfeedback indicator field value. To support this approach, the PMIindicator for both the downlink and uplink signaling needs 2-bits.

As an alternative, the codebook shown in FIG. 7 can be substituted forthe various codebooks shown above in FIG. 4 or 5. Instead of using thepreviously-identified codebooks in FIGS. 4-7, the present invention alsosupports the use of codebook subsets shown in FIGS. 8 and 9, either ofwhich can be used in the above configurations. That is, the codebooks inFIGS. 8 and 9 can be selected using two configurations.

In one configuration, the attachment point (base station/antenna) mayselect one of the two subsets shown in FIG. 7, and either FIG. 8 or 9for use in a sector where the user is located. The attachment pointselects a subset codebook for all user equipment in the same sector,such as using only the codebook shown in either FIG. 8 or 9. Theattachment point selects the codebook subset for the user equipmentbased on some knowledge of the user equipment's channel condition. Thechannel condition information includes information regarding the userequipment's location information, the error rate for transmissions tothe user equipment, the number of re-transmissions to the userequipment, and the uplink sounding or other uplink transmissions, withthe uplink received beam-forming using a similar beam pattern as thatfor the downlink transmission.

In a second configuration, the user equipment can select the appropriatecodebook subset to be used in either FIG. 8 or 9, and the user equipmentcan select between the different distinct codewords for a twotransmission antenna (2-Tx) system. The user equipment transmits anindicator that implicitly or explicitly indicates which codebook subsetis chosen. The subset selection will be dictated in the secondconfiguration through a higher layer activation depending on thecodeword selected from the codewords shown in FIG. 7, and either FIG. 8or 9, and the index of the selected codeword in the subset is signaledusing 2 bits through the normal PMI feedback indicator field value. Tosupport this approach, the PMI indicator for both the downlink anduplink signaling needs 2-bits.

Further, the attachment point may also use a larger codebook subsettable as shown in FIGS. 10 and 11 for use in a sector where the user islocated. The attachment point selects a codebook for all user equipmentin the same sector, such as using only the codebook shown in FIG. 10 or11. To support this approach, the original codebook with antennaselection codewords will be optimized using 3 bits, and the PMIindicator for both the downlink and uplink signaling needs 3-bits toallow the proper selection of the increased number of codewords. Theselection of the codebook subset for this configuration can also beconfigured using the Radio Resource Configuration (RRC) signaling, whichcan select the use of codebooks in FIG. 10 or 11 instead of otherdefault codebook subsets set by the system. The attachment point mayalso select the codebook subset for the user equipment based on someknowledge of the user equipment's channel condition. The channelcondition information includes information regarding the userequipment's location information, the error rate for transmissions tothe user equipment, the number of re-transmissions to the userequipment, and the uplink sounding or other uplink transmissions, withthe uplink received beam-forming using a similar beam pattern as thatfor the downlink transmission.

The application of the signal strength weightings can also be optimizedusing an antenna beam indicator. The indicator may be a field in theuplink or downlink transmission packets. The length (number of bits) forsuch an indicator will depend on the number of available antennas in thenetwork location. One bit length is sufficient for two antennaarchitectures, while 2 bits is sufficient to designate up to fourantennas. The antenna beam indicator can also be designated according toa bit map with each bit identifying one of the available beams that canbe used to communicate with the user equipment.

Based on the specific beam location, the user equipment will provide anindicator bit value or bit map value indicating which beam can providethe best coverage for that user equipment. The use of that antenna beamindicator over a specific period of time will depend on the userequipment mobility, with the indicator being valid longer for slowermoving user equipment and being valid for a shorter period of time forfaster moving user equipment. Thus, the antenna beam indication needs tobe updated with a periodicity corresponding to the changes.

The use of an antenna beam indicator is made possible through theestimation of the uplink transmission condition, such as an analysis ofthe sounding, random access, or other types of uplink transmissions fromthe user equipment. The access point may also use a direction-findingalgorithm to determine the beam index for user equipment using the SDMAprotocols. The CQI index can be used to provide selection information tothe access point, which can also analyze the signal-to-interference andnoise ratio and identification of the serving beam for the userequipment.

In systems with switching beams or opportunistic beams (e.g. OSTMA), theuser equipment provides a CQI index when it is within the coverage areaof a beam that has been switched (powered) on. Based on the time whenthe CQI is received by the access point, the beam index can beimplicitly determined because the beam pattern is known by the accesspoint.

The technology as described above allows the configuration of additionalcodebooks for UE feedback in closed-loop operations, so that a moreappropriate codebook can be used to support different antennaconfigurations, e.g. correlated, uncorrelated or cross-polarized antennasystems. To allow the support of various antenna configurations thatwould be favorable for different deployment scenarios, e.g., correlated,uncorrelated or cross-polarized antenna systems, LTE-Advanced maysupport additional codebooks to be used for UE feedback in closed-loopoperations. For backward compatibility, higher-layer (RRC) signaling canbe used to configure the use of a different codebook by some or all ofthe UEs conveniently, depending on the UE capability, e.g., Rel1-8 UEsor LTE-A UEs, and the deployment configuration, e.g., correlated,uncorrelated or cross-polarized antenna systems. As the codebook isconfigurable, the larger UE-specific codebook can be configured when ahigher capacity is required in the deployed system. Otherwise, thesmaller codebook can be used to minimize UE complexity.

While the foregoing has been with reference to a particular embodimentof the invention, it will be appreciated by those skilled in the artthat changes in this embodiment may be made without departing from theprinciples and spirit of the invention, the scope of which is defined bythe appended claims.

What is claimed is:
 1. A method for operating a first communicationstation to facilitate communication between the first communicationstation and a remote communication station, wherein the firstcommunication station includes a plurality of antennas, the methodcomprising: acquiring information about the channel condition betweenthe first communication station and the remote communication station;selecting a codebook weighting matrix for the remote communicationstation from a designated subset of a codebook based on an analysis ofdata including the channel condition information, wherein the codebookincludes at least six codebook weighting matrices, wherein each of thecodebook weighting matrices includes one or more layer-related columns,wherein each of the layer-related columns includes a plurality ofcomplex weight values having absolute value less than or equal to one;precoding one or more layer signals based on the selected codebookweighting matrix to obtain transmit signals for the respective antennas;and transmitting the transmit signals respectively through the antennas.2. The method of claim 1, wherein the complex weight values of thecodebook weighting matrices in the designed subset have constantmodulus.
 3. The method of claim 1, wherein, in one or more cases, saidselection of the codebook weighting matrix is determined by the channelcondition information.
 4. The method of claim 1, wherein, in one or morecases, said selection of the codebook weighting matrix overrides acodebook weight matrix indicated by the channel condition information.5. The method of claim 1, wherein the at least six codebook weightingmatrices include at least one antenna selection codeword.
 6. The methodof claim 1, wherein said acquiring information about the channelcondition is based on an application of a direction finding algorithm totransmissions from the remote communication station.
 7. The method ofclaim 1, wherein the designated subset consists of six codebookweighting matrices, wherein the complex weight values of the sixcodebook weighting matrices have constant modulus, wherein, for a givensector in which the remote communication station resides, the firstcommunication station is configured so that all UEs in the given sectorthat are configured for two-antenna based PMI feedback are constrainedto use codebook weighting matrices from within the designated subset. 8.The method of claim 1, wherein the designated subset consists of sixcodebook weighting matrices, wherein the complex weight values of thesix codebook weighting matrices have constant modulus, wherein all UEsin the same sector as the remote communication station are required touse the designated subset.
 9. A base station to facilitate communicationwith a remote communication station, the base station comprising: aplurality of antennas; and circuitry configured to: acquire informationabout the channel condition between the base station and the remotecommunication station; select a codebook weighting matrix for the remotecommunication station from a designated subset of a codebook based on ananalysis of data including the channel condition information, whereinthe codebook includes at least six codebook weighting matrices, whereineach of the codebook weighting matrices includes one or morelayer-related columns, wherein each of the layer-related columnsincludes a plurality of complex weight values having absolute value lessthan or equal to one; precode one or more layer signals based on theselected codebook weighting matrix to obtain transmit signals for therespective antennas; and transmit the transmit signals respectivelythrough the antennas.
 10. The base station of claim 9, wherein thecomplex weight values of the codebook weighting matrices in the designedsubset have constant modulus.
 11. The base station of claim 9, wherein,in one or more cases, said selection of the codebook weighting matrix isdetermined by the channel condition information.
 12. The base station ofclaim 9, wherein, in one or more cases, said selection of the codebookweighting matrix overrides a codebook weight matrix indicated by thechannel condition information.
 13. The base station of claim 9, whereinthe at least six codebook weighting matrices include at least oneantenna selection codeword.
 14. The base station of claim 9, whereinsaid acquiring information about the channel condition is based on anapplication of a direction finding algorithm to tranmsissions from theremote communication station.
 15. The base station of claim 9, whereinthe designated subset consists of six codebook weighting matrices,wherein the complex weight values of the six codebook weighting matriceshave constant modulus, wherein, for a given sector in which the remotecommunication station resides, the circuitry is configured so that allUEs in the given sector that are configured for two-antenna based PMIfeedback are constrained to use codebook weighting matrices from withinthe designated subset.
 16. The base station of claim 9, wherein thedesignated subset consists of six codebook weighting matrices, whereinthe complex weight values of the six codebook weighting matrices haveconstant modulus, wherein the circuitry is configured so that all UEs inthe same sector as the remote communication station are required to usethe designated subset.
 17. A non-transitory memory medium for operatinga first communication station to facilitate communication between thefirst communication station and a remote communication station, whereinthe first communication station includes a plurality of antennas,wherein the memory medium stores program instructions, wherein theprogram instructions, when executed by a processor, cause the processorto implement: acquiring information about the channel condition betweenthe first communication station and the remote communication station;selecting a codebook weighting matrix for the remote communicationstation from a designated subset of a codebook based on an analysis ofdata including the channel condition information, wherein the codebookincludes at least six codebook weighting matrices, wherein each of thecodebook weighting matrices includes one or more layer-related columns,wherein each of the layer-related columns includes a plurality ofcomplex weight values having absolute value less than or equal to one;precoding one or more layer signals based on the selected codebookweighting matrix to obtain transmit signals for the respective antennas;and transmitting the transmit signals respectively through the antennas.18. The memory medium of claim 17, wherein the complex weight values ofthe codebook weighting matrices in the designed subset have constantmodulus.
 19. The memory medium of 17, wherein the designated subsetconsists of six codebook weighting matrices, wherein the complex weightvalues of the six codebook weighting matrices have constant modulus,wherein, for a given sector in which the remote communication stationresides, the first communication station is configured so that all UEsin the given sector that are configured for two-antenna based PMIfeedback are constrained to use codebook weighting matrices from withinthe designated subset.
 20. The memory medium of claim 17, wherein thedesignated subset consists of six codebook weighting matrices, whereinthe complex weight values of the six codebook weighting matrices haveconstant modulus, wherein the first communication station is configuredso that all UEs in the same sector as the remote communication stationare required to use the designated subset.